To use better the lumber produced by a modern mill, it is desirable to have some indication of the structural value of each board produced.
Means for Measuring Stiffness of Lumber, patented by Harold A. Keller in 1965, U.S. Pat. No. 3,196,672, has formed the basis for a machine, known as the CLT Continuous Lumber Tester (CLT), used commercially in production lines for the production of machine stress rated (MSR) lumber. The CLT is responsible for most of the production-line testing of lumber stiffness in North America and, as production speeds increase in other parts of the world, the CLT is increasingly being used there as well. The CLT non-destructively obtains measurements of bending stiffness, which it relates through lumber cross-section size and bending span length to the material property modulus of elasticity (E) for each wood board passing through the machine. Strength correlates with E sufficiently well that E is used as an indicator of strength as well as being important in its own right.
The CLT, positioned typically at the output of a high-speed planing unit, accepts lumber boards from the planer, bends each board by a fixed deflection, first downward in a first test span and then upward in a second test span, averages the forces required at each of the two test spans to eliminate effects of board weight and straightness deviations, then after signal processing, marks each board with an ink spray mark to indicate the E category.
The CLT is built on a framework to which are mounted three fixed clamp carriages spaced along the framework and bolted to it. Each fixed clamp carriage contains four rollers, two of which are 8.5 inches 215.9 mm! diameter and two of which are slightly smaller than 4.5 inches 114.3 mm! diameter. All six of the large (8.5 inch) clamp rollers, two in each of three fixed clamp carriages, are driven by two motors. Each motor drives three rollers via sprockets on the ends of the roller shells that are connected together for motive purposes by timing belts.
Three additional clamp carriages are movable with respect to the CLT framework. The motion of each of these movable clamp carriages is guided translationally in the vertical direction by slide bearings and by a rack and pinion assembly that together prevent any significant rotation of the movable clamp carriage about either a vertical axis or an axis parallel to the lumber flow. The number, size and arrangement of rollers in the movable clamp carriages are the same as in the fixed clamp carriages, except the movable clamp carriages are located on the opposite side of the lumber from the fixed clamp carriages.
Each movable clamp carriage is positioned so that its rollers press the lumber against rollers in a corresponding fixed clamp carriage. Force to lift and press the movable clamp carriages against the lumber and thence against the fixed clamp carriages is provided by air powered actuators, one for each movable clamp carriage.
Thus, in the CLT each of the fixed and movable clamp carriages contains two large and two small rollers. For all four clamp rollers in a clamp carriage to contact a lumber surface on a common plane, the small rollers have axes that are displaced from the large rollers in a direction perpendicular to the contacted lumber surface. In the CLT this displacement distance is two inches 50.8 mm!. With 8.5 inch 215.9 mm! and 4.5 inch 114.3 mm! diameters for the large and small rollers respectively, and the above displacement, the rollers would all be tangent to a common plane. This would be true for the rollers in each of the fixed clamp carriages on one side of the lumber and also for the opposed rollers in the corresponding movable clamp carriages on the other side of the lumber. However, machine manufacturing tolerances and discrepancies in thickness along a wood board would lead to situations where not all rollers contact the board on both sides simultaneously. Because only the two large rollers in each fixed clamp carriage are driven, there are conditions where contact of these drive rollers to the board surface and hence motive force for propelling the board through the machine would be lost.
To avoid losing motive force in the CLT, the small clamp rollers are reduced in size by a small amount to ensure roller contact with the wood board surface by the large driven rollers under all reasonable conditions. Although some clamping pressure at the small rollers is lost, this tradeoff is essential because of the serious consequences of measurement inaccuracy if the speed of the board is not maintained through the machine.
The three combinations of movable and fixed clamp carriages isolate two bending test sections from effects of forces external to the test sections. A total of 24 clamp rollers clamp lumber between the movable and fixed clamp carriages, control the position of the lumber at the ends of the two test sections and propel it through the machine. At the center of each test section, a load roller forces a fixed deflection of the lumber, and the force required to achieve this deflection is sensed by a strain gauge type load cell. The two test sections are similar, except that in one test section the lumber is bent in one direction, and in the other it is bent in the other direction. By delaying the force signal from the load cell in the first test section by the amount of time required for a point on a wood board to move from the first to the second test section, the two force signals from a common location on the board can be combined to form one-half their sum. The result, known as local E, is compensated for warp of the wood board.
Lumber enters the CLT through an infeed guide, recently modified with an airoperated mechanism to allow easier clearing of material jams. Each board is gripped between the rollers of fixed and movable clamp carriages and propelled into and through the machine by the large driven clamp rollers in the fixed clamp carriages.
Adjustable stop bolts attached to the movable clamp carriages limit the movement of the movable clamp carriages and establish the opening between rollers in the fixed and movable clamp carriages when lumber is not in the CLT. If stop bolts are set for too large an opening, the movable clamp roller carriages do not move enough to grip the lumber adequately against the fixed clamp carriages or to bend the lumber by the amount required to achieve accurate readings. In this case, motive force may be lost, and a reduction in grade yield will occur. When the stop bolts are set for too small an opening, the movable clamp carriages will be closer than they should be to the fixed clamp carriages when boards are not in the machine. This leads to more translational motion than desirable for the movable clamp carriages as boards enter and exit the machine, thus causing increased machine wear, but most importantly, measurement errors, i.e. inertial noise, due to the inertial effects of accelerating masses. Compensation for inertial noise has been introduced to the CLT E measurement process, and inertial noise compensation is disclosed in U.S. Pat. No. 4,991,446; but, it is better to minimize the motion and avoid introducing the inertial noise in the first place by careful stop bolt adjustment rather than to compensate for the inertial noise errors in the measurement once they have been introduced.
Steel guides (recently improved in the CLT), one located in the first test section and another in the second test section, guide the leading ends of boards smoothly in the direction of bending between the rollers of fixed and movable clamp carriages as the boards enter and exit the spaces between these pairs of carriages. These guides help reduce mechanical shock and vibration and thus improve measurement accuracy by reducing inertial noise.
Two opposed, longitudinal fences extend throughout the length of the CLT, forming a channel down the centerline of the CLT to control the position of wood boards laterally in a direction perpendicular to the direction of bending. These fences are pressed by air powered actuators in a direction toward the CLT longitudinal centerline against adjustable stops to accommodate lumber having different lumber widths.
In each test section, a load roller deflection assembly bends the lumber by a fixed amount. CLT load roller assemblies are comprised of two load roller halves on a shaft, a means by which the load roller can rock about a longitudinal axis of the machine to track twist in the surface of lumber traveling through the CLT, and a supporting structure by which it is mounted at one end with flange pivot bearings or, more recently, by flexure couplings (U.S. Pat. No. 4,932,267) to the CLT frame.
In the CLT, lumber is bent downward in the first test section and then upward in the second test section. In each test section, the force required for bending is measured by a load cell and force measuring system, and the upward and downward force measurements are averaged point-by-point along the length of the lumber to obtain "local E" as a function of position along the length. The force signal from the load cell in the first test section is delayed in an electronic data processing unit before averaging with the force signal from the second test section. The delay, equal to the amount of time required for lumber to travel from the first to the second test section, is required so that downward and upward forces are combined (averaged) at the same points along the lumber. This local E measurement is processed in the electronic data processing unit to obtain an average value "Average E" and a lowest value "Low-point E" over the length of the lumber. Average E and Low-point E are used in the United States and Canada to determine an E category and define an ink spray color mark that is applied automatically to the lumber. In some countries, only the Low-point E is used and in some countries spray marks identifying local E are applied along the length of the boards.
Photosensors detect the ends of the wood boards and thereby control timing of the Average E and Low-point E computational process as well as timing of E category determination and application of ink spray marks.
Three major features of the CLT retained by the present disclosure are: 1. two test sections and signal averaging to compensate the E measurement for warp of a board; 2. fixed-deflection force measurement (as opposed to fixed-force deflection measurement) to reduce errors in the measurement caused by inertial noise; and 3. multiple clamp roller supports at ends of test sections to isolate measurements from effects of external forces.